1. Field of the Invention
This invention pertains generally to monitoring strain, and more particularly to using strain monitoring as an indicator of medical conditions including monitoring the progress of spinal fusion, monitoring glucose levels, measuring spinal loading, and monitoring heart rate.
2. Description of Related Art
Lumbar fusion is one of the fastest growing areas of orthopedic surgery.
The most common indication for surgical intervention is pain in the lower back. Although many devices have been designed to minimize the incidence of work-related back injury, as a society we still participate in many activities that lead to back injury. Most frequently, inappropriate lifting of objects, pulling or lifting objects from awkward angles, and fatigue lead to injury of the back muscles. If the level of injury is severe enough, the muscles and ligaments of the lumbar spine cannot withstand the load applied, and the intervertebral disc will become herniated from the anterior side of the spine. This is often called a herniated or ruptured disc. In addition, the vertebrae of the spine articulate with each other through the transverse and spinous processes located on the posterior aspect of the vertebrae. In between the processes, there are small pads of cartilage that can become damaged with a back injury. Both herniated discs and the processes can cause chronic pain and loss of function in the spine. Pain results in debilitation and prevents the patient from enjoying ordinary daily activities.
To eliminate the pain, a lumbar fusion is performed wherein an incision is made over the lumbar region of the spine and metal bracing is applied bilaterally to the posterior of the vertebrae. This bracing provides initial mechanical stiffness until bone growth, stimulated by a bone growth factor, encapsulates the metal bracing and eliminates motion between the two lumbar vertebrae. There are many choices for the metal bracing, called spinal instrumentation, which can be used to create the initial fixation. In general, a pedicle screw is screwed from the posterior through the pedicle bony bridge of the vertebrae and into the wall the vertebral body. This procedure is repeated for the neighboring vertebrae and bilaterally on the opposite side of the posterior spine. Once all four pedicle screws are in place, a rod or plate is placed over posts on two of the pedicle screws. The rod or plate is then held down with locking nuts that screw onto the posts. A slurry of bone and bone growth factor is applied over the spinal instrumentation and vertebrae, and the incision is closed.
After lumbar fusion surgery, rehabilitation takes several months. The patient is immobilized with a brace that extends from beneath the arms to midline of the hips and is instructed not to perform any strenuous physical activity. No lifting, driving, running or bending at the waist is allowed. Any kind of activity that involves impact is also prohibited, such as roller coasters. The patient must wear the brace until fusion is visible on an x-ray radiograph. Depending on the age of the patient, this can be anywhere from four months to a year after surgery. Because of this extended period of immobility, the muscles of the spine and abdomen atrophy from disuse. The brace also contributes to stress shielding, meaning the brace is carrying some of the spinal load, resulting in an inferior strength lumbar fusion.
The problem with the foregoing treatment approach, is that fusion occurs much sooner than is predicted by radiographs. For example, a solid fusion could occur as early as eight weeks (two months) after surgery. However, the bone that initially grows around the spinal instrumentation is trabecular bone, and although it is strong and dense, it is not radiographically opaque. Thus, it cannot be seen on an x-ray until it has been infused with minerals, such as calcium.
There are several methods for measuring the movement or strain in the human spine, including those that involve collecting an electronic signal and transmitting it to an external receiver. For example, U.S. Pat. No. 6,433,629 teaches using a Wheatstone bridge and a timing circuit to measure the displacement (strain) in an orthopedic implant. In addition, the device does not use an internal power source. Instead, a magnetic coil brought in close proximity to the Wheatstone bridge provides power to the circuitry and activates the circuitry for the duration of the measurement.
In U.S. Pat. No. 5,935,086, the relative angles between two or more joint are measured and a force transducer is used to simultaneously measure the applied force in the joint of an artificial knee. This is similar to U.S. Pat. No. 5,995,879, which also measures the angle between two freely movable points to determine the orientation of a second spinal vertebrae relative to a first vertebrae.
U.S. Pat. No. 6,432,050 uses audible acoustic feedback to monitor an in vivo sensor or device. By applying an acoustic query to the implanted device, the operator can audibly determine if the device is functioning properly. This has wide reaching applications, from heart surgery stents, to intervertebral disc implants.
In U.S. Pat. No. 6,223,138, a Wheatstone bridge is used to measure strain displacement, but the signal is amplified and added it to a carrier frequency. By adding the signal to a secondary frequency, loss of a small signal in the background noise is avoided.
Published U.S. patent application number US2002/0050174 A1 also uses strain gages in a Wheatstone bridge, the device has been adapted to successfully measure strains on the micron scale.
Published U.S. patent application number US2004/0011137 A1 also provides information concerning the current state of the art.
Each of the foregoing U.S. patents and published patent applications is incorporated herein by reference in its entirety.
Notwithstanding the foregoing approaches to measuring strain, the onset of spinal fusion after lumbar surgery continues to be difficult to determine, and patients are frequently fitted with a spinal brace for three to six months after surgery even though the implant provides internal fixation in a much shorter period of time. If a new method could be developed that could detect a solid fusion without the need for radiographic verification, the amount of time patients would need to be in a brace could be cut by 50% or more.
Similarly, there is a need for new approaches to monitoring strain in other parts of the body and for monitoring other medical conductions. The present invention satisfies those needs and advances the state of the art.